This invention relates to optical fiber connectors, and, more particularly, to connectors which are capable of connecting an optical fiber to a source or to another optical fiber with very little sensitivity to lateral misalignment.
Although the present invention finds utility in the coupling of light from a source to an optical fiber, the present discussion concerning connector alignment problems will be limited to fiber-to-fiber connectors. The butt connection between the ends of two fibers will result in an insertion loss that is caused by various optical fiber misalignment parameters. Examples of such misalignment parameters are as follows: (a) lateral misalignment between the axes of the two fibers, (b) longitudinal misalignment between the endfaces of the two fibers, and (c) angular misalignment between the axes of the two fibers.
The usual approach to fiber coupling is the butted end coupler in which fibers which have had their endfaces prepared are brought into angular, lateral and axial alignment. Such alignment is difficult to achieve outside the laboratory. Since the butted fiber arrangement is particularly sensitive to lateral displacement, this type of connector is difficult to use in field applications.
Beam expanders employing lenses or tapered fibers have been employed in in-line single-mode fiber connectors which, due to the small core diameter of such fibers, are extremely sensitive to lateral misalignment. Although such beam expanders exhibit a reduced sensitivity to lateral displacement, they are generally very sensitive to angular misalignment. The art of aligning two connector halves is sufficiently advanced that angular misalignment is not a primary concern. Expanded beam connectors are therefore receiving a considerable amount of attention.
The basic principal of expanded beam connectors is described in the publication K. P. Jedrzejewski et al. "Tapered-Beam Expander for Single-Mode Optical-Fiber Gap Devices", Electronics Letters, 16th Jan. 1986, vol. 22, No. 2, pp. 105-106. That publication teaches a connector of the type wherein the end of a single-mode fiber having a core refractive index n.sub.1 and a cladding refractive index n.sub.2 is inserted into a tube of glass having a refractive index n.sub.3 which is lower than n.sub.2. The refractive index of the tube is slightly lower than cladding n.sub.2. The capillary tube is uniformly heated to collapse it about the fiber. The central region of the combined fiber and capillary is then tapered to a minimum neck diameter of 40 .mu.m, which is appropriate for fiber handling and cleaving. A taper ratio of 4:1 is said to be adequate for minimizing insertion loss. Since the end of the fiber forms the central portion of the connector, no means need be provided for attaching the fiber to the connector half. However, that fiber cannot be easily disconnected from the connector half and replaced by another fiber. Furthermore, by employing the fiber as the internal portion of the connector half, the maximum spot size is limited.